The invention relates to a radial compressor, and more specifically, a radial compressor with wall slits.
Radial compressors are used in many fields of technology for producing a compressed gas flow. Thus, for example in many internal combustion engines, in particular in larger diesel engines, the charge air supplied is compressed by means of a radial compressor in order to increase the output. This charge-air compression is often designated as xe2x80x9csuperchargingxe2x80x9d. The radial compressor is often driven by an exhaust-gas turbine and forms with the latter a so-called exhaust-gas turbocharger.
In radial compressors there is the problem that they assume an unsteady operating state if the volumetric flow falls below a certain value. In this connection, the term xe2x80x9csurgingxe2x80x9d of the radial compressor is also used. In this state, the radial compressor builds up only an inadequate pressure, a factor which, in supercharged internal combustion engines for example, becomes apparent as a so-called xe2x80x9cturboholexe2x80x9d at low speeds. In addition, pronounced unsteady flows occur which mechanically load the components of the radial compressor to a considerable degree and can reduce the service life of the radial compressor.
Various measures have therefore been developed, the aim of these measures being to reduce the surge limit, i.e. the limit between the steady and the unsteady range, towards lower volumetric flows. In supercharged internal combustion engines, this increases their output in the low speed range and leads to reduced pollutant emissions.
One of these measures is known as so-called wall treatment and includes the making of wall slits in the casing wall adjoining the flow duct, as described in U.S. Pat. No. 4,212,585 establishing the generic type. The radial compressor disclosed in this publication has a casing in which a hub provided with moving blades is arranged. An inner surface of the casing and the surface of the hub form the boundary surfaces of a flow duct. Arranged therein concentrically to the hub is a ring which is accommodated in a recess in the inner surface of the casing. That end face of the ring which points downstream abuts so snugly against a corresponding end face of the recess that the flow duct has no step on the casing side. At the height of the moving-blade ends directed upstream, wall slits are arranged in the inner wall of the ring, and these wall slits produce the desired displacement of the surge limit. It is assumed that the wall slits counteract the formation of recirculation vertices which form in the front impeller region and are considered to be the reason for the start of the xe2x80x9csurgingxe2x80x9d.
Laid-open specification EP 0 348 674 A1 discloses a device which is intended for extending the characteristic diagram of a radial compressor in the direction of small rates of flow in the inlet region of the impeller of the compressor and contains a recess which runs in the circumferential direction of a compressor inlet duct and extends upstream from the inlet opening of the impeller, and in which a stabilizing ring is integrated, the stabilizing ring being arranged in front of the impeller and outside the main flow duct and carrying a number of blades on its outer circumference, the blades in turn being anchored to the inner contour of the recess.
Laid-open specification DE 40 27 174 A1 describes a radial compressor having a device for stabilizing the characteristics, this device having a circulation space for the purpose of balancing the pressure between compressor impeller and an inlet region upstream of the impeller. Provided in the inlet region is an intake ring with which the flow is to be influenced there in such a way as to stabilize the characteristics and minimize the losses and which can be modified in a customized manner. The circulation space is defined radially on the inside by a contour ring, which extends axially between an upstream intake groove connected to the inlet region and a contour groove which opens into the main flow in the region of the impeller contour.
The object of the invention is to provide a radial compressor of the type mentioned at the beginning in which a reduction in the surge limit is achieved by suitable measures.
A radial compressor according to this invention has a casing having a recess formed in an inner surface. An impeller is arranged in the casing for rotation about a rotational axis. The impeller has a hub carrying blades, with the hub and inner surface of the casing defining a flow duct. A ring is received in the recess formed in the inner surface of the casing and arranged concentrically with the hub of the impeller. The ring has slits in an inner surface in a region spaced axially from a downstream facing end face of the ring. The downstream facing end face of the ring is in spaced axial relation to an upstream facing wall of the recess, defining an annular gap therebetween. The blades of the impeller have upstream facing ends that lie axially between opposed axial ends of the wall slits, preferably at a distance from the two opposed axial ends of the wall slits that is greater than the wall-slit width. It has been found that, in radial compressors designed in such a way and having a gap, the surge limit is once again clearly displaced towards lower volumetric flows. Especially good results can be obtained if, to create a pressure balance, the gap is connected to the upstream space in front of the ring via passages which are independent of the flow duct.
In a further embodiment, the extent or width of the annular gap in the axial direction is 1 to 5 per cent of the diameter of the impeller. In a further embodiment, the wall slits extend obliquely from the inner surface of the ring at an angle of 30xc2x0 to 90xc2x0 to the inner tangent of the ring into the transverse plane of the latter down to a depth which is approximately equal to the sound velocity of the gas divided by the product of four times the rotational frequency and the number of blades on the impeller. In this case, the extent of the wall slits in the axial direction is not greater than this depth. It has been found that each of these development contributes to a low surge limit.
In a further embodiment, the ring in the recess is guided in the radial direction by longitudinal ribs arranged on the inside of the casing. The longitudinal ribs, together with the casing inner surface and the ring outer surface, form the passages already mentioned above for creating a pressure balance between the gap and the upstream space in front of the ring. No additional bores in the casing are therefore necessary.
In a further embodiment, the ring is fixed in the axial direction by one or more screws passed through the casing wall. This fastening is simple in terms of design and can be released at any time without problem.
In a further embodiment, the ring is arranged in the recess so as to be displaceable in the axial direction. It is possible by suitable fixing means to fix the ring in several different axial positions. In this way, the extent of the gap in the axial direction, i.e. the gap width, can be varied without new components or extensive modifications being necessary for this purpose. An adjustment of the gap width may be appropriate, for example, in order to adapt the radial compressor to changed operating parameters, for instance temperature and density of the intake air.